Method for manufacture of a cold-gas pyrotechnic generator

ABSTRACT

A method of manufacturing a cold-gas pyrotechnic generator of the type having a casing including a base and a cover with an internal chamber accessed by a central opening for the reception of pyrotechnic elements includes the steps of placing the cover on the base with filtering material inserted, rotating the casing while introducing the pyrotechnic elements through the central opening and then inserting an ignition device in the central opening.

This is a division of application Ser. No. 07/544,511, filed Jun. 27,1990 now U.S. Pat. No. 5,139,280.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of automobile safety and inparticular the protection, in the event of an impact, of the passengersof a motor vehicle by inflation of a protective pillow by means of thecombustion gases originating from a cold-gas pyrotechnic generator. Tobe more precise, the invention relates to a cold-gas generator of thistype with welded coaxial chambers and a method of manufacture applicableto a generator of this type enabling it to be loaded with pyrotechnicmaterial after the mechanical components forming the various chambershave been welded.

2. Description of Related Art

It has been envisaged for many years now to protect the passengers of amotor vehicle, in the event of an impact, by inflation of a pillow whichcomes between the body of the passenger and the dashboard of thevehicle. Inflation of the pillow is ensured by a gas source released byan acceleration or deceleration detector calibrated above a certainthreshold. In order to ensure the reliability of the protective deviceover time, the gas source must both preserve well over time and have avery short response time, of the order of only a few tens ofmilliseconds.

These two requirements have led a person skilled in the art to prefer asthe gas source cold-gas pyrotechnic generators in the form of metalcasings which consist essentially of a base and a cover defining insidethe casing an ignition chamber which contains an ignition deviceconnected to the acceleration or deceleration detector, a combustionchamber which contains a gas-generating pyrotechnic composition, and oneor more chambers for filtering or cooling the gases. Since these gasgenerators must have a reduced size, and in particular a reduced height,so as to be able to be housed in particular in steering wheels of motorvehicles, a person skilled in the art generally adopts a cylindricalshape for these generators, the ignition chamber being central and thecombustion and filtering chambers having an annular shape and beingarranged either inside each other or on top of each other, thecombustion chamber surrounding all or part of the ignition chamber.Generators of this type are, for example, described in U.S. Pat. Nos.4,547,342, 4,578,247 or 4,590,041.

The pyrotechnic composition employed is generally a mixture of sodiumnitride and mineral oxidizing agents. Since sodium nitride is anexplosive compound sensitive to humidity, it is recommended that the gasgenerator be sealed tightly and so that it cannot be removed, on the onehand in order to protect the composition from humidity and on the otherhand to prevent any inopportune feedback of the explosive component. Forthese reasons, the base and the cover forming the casing of thegenerator are usually welded together, it being possible for welding tobe carried out advantageously by friction or by inertia, as indicated inthe patents cited hereinabove.

However,this solution has a serious disadvantage. Most of the designsadopted up until now for gas generators do not allow them to be filledwith pyrotechnic material after welding and it is thus necessary to fillthem with pyrotechnic material before welding and to carry out thisoperation in the presence of pyrotechnic material which contains a highdegree of sodium nitride, an operation which is far from being free ofdanger.

It is known to the Applicant that an attempt has been made to makepossible this type of filling. This attempt is described in U.S. Pat.No. 4,249,673. This patent describes a gas generator of revolutionformed by a base and a cover establishing two concentric chambers, aninner ignition and combustion chamber and a peripheral annular coolingand filtering chamber. The base of the generator has a central orificeto which it is possible, after the generator has been assembled, tointroduce pellets of a pyrotechnic composition, and then the ignitiondevice, into the inner chamber. Although the solution described in thispatent allows the filling of the generator with pyrotechnic materialafter the base and the cover have been assembled, it nevertheless hastwo other serious disadvantages. So as to be able to ensure filling ofthe pellets and introduction of the ignition device, the generator mustbe given a height which is very much greater than that of the igniter.This makes it impossible to meet the standards imposed by constructorsof motor vehicles who demand "flat" generators, whose height barelyexceeds that of the igniters. Moreover, after loading, the pellets of apyrotechnic composition are arranged around the igniter and above thelater, which makes it impossible to ensure the symmetry of the ignitionand the run-off of the gases into the generator where symmetry isnecessary if very short response times are desired without risking anover-pressure in the generator.

At the present time a person skilled in the art does not therefore haveat his disposal a gas generator having good symmetry of ignition andrun-off of the gases and capable of being loaded safely with pyrotechnicmaterial after the constituent elements of its casing have been welded.

SUMMARY OF THE INVENTION

The object of the present invention is precisely to provide such agenerator for a person skilled in the art, as well as a method formanufacturing and filling this generator.

The invention therefore relates to a cold-gas pyrotechnic generatorcomprising a casing of revolution provided with outer orifices andconsisting of a base and a cover fastened together transversely, thesaid base and the said cover establishing:

an internal central ignition channel in which an ignition device isplaced,

an annular intermediate chamber containing elements of a gas-generatingpyrotechnic composition,

a chamber containing filtering means and communicating with the outsideof the generator through the said outer orifices,

the said chambers being in communication through intermediate orifices,the said generator being characterized in that the said casing ofrevolution defines, between the said inner central channel and the saidannular intermediate chamber, a lateral opening whose height is greaterthan the largest dimension of the said elements of a gas-generatingpyrotechnic composition.

The said lateral opening is preferably continuous.

According to a preferred embodiment of the invention, the said base has:

an outer lateral wall,

a cylindrical, intermediate initial partition section,

a central circular opening bordered by a cylindrical, inner initialpartition section,

and the said cover in turn has:

an outer lateral wall which has the said outer orifices,

a cylindrical intermediate partition which has the said intermediateorifices, this partition having the same diameter as the intermediateinitial partition section carried by the said base,

a cylindrical, inner initial partition section with a diameter slightlyless than the diameter of the inner initial partition section carried bythe said base,

the said lateral opening being arranged between the inner initialpartition section carried by the said base and the inner initialpartition section carried by the said cover.

The said elements of a gas-generating pyrotechnic composition areadvantageously immobilized by an expansible immobilization means whichmay consist of a wire mesh wound around itself and placed so as to bearagainst the inner initial partition sections carried by the said baseand by the said cover.

According to another preferred embodiment of the invention, the saidcover has a central circular opening with a diameter slightly less thanthe diameter of the central circular opening carried by the said base,the said central circular opening carried by the cover being bordered bythe said inner initial partition section carried by the base, the innerlateral surface of the said inner initial partition section beingthreaded.

According to a final preferred variant of the invention, the saidignition device comprises on the one hand a ring which can be screwedinto the said inner central channel, the said ring having over itsentire lateral surface slots which can face the said lateral opening andbring the said annular intermediate chamber into communication with thesaid inner central channel, and on the other hand a pyrotechnic igniterwhich can be fastened inside the said ring.

The invention also relates to a method for manufacturing a cold-gaspyrotechnic generator of this type, a method characterized in that:

i) in a first stage, the said casing is formed by assembling the saidcover and the said base containing the said filtering means,

ii) in a second stage, the casing thus formed is fastened onto arotating device and, after the casing has been set in rotation, theelements of a pyrotechnic composition are poured into the said lateralopening of the said annular intermediate chamber inside which they areheld by the centrifugal force communicated to the casing,

iii) in a third stage, the immobilization means, as well as the ignitiondevice, are introduced into the inner central channel.

According to a preferred variant embodiment of the invention, theassembly of the base and the cover is carried out by welding, and inparticular by friction welding or by inertia welding.

According to another preferred variant of the invention, in the secondstage the casing is fastened onto a horizontal-spindle lathe, the speedof rotation of which, during the loading of the casing, is greater than100 revolutions per minute.

Lastly, according to a final preferred variant, during loading, thecasing is subjected to vibrations.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the invention is given hereinafter withreference to FIGS. 1 and 2 which illustrate the preferred embodiment ofthe invention.

FIG. 1 shows, seen in cross-section, a cold-gas pyrotechnic generatoraccording to the invention,

FIG. 2 shows, also seen in cross-section, the casing of the generatorshown in FIG. 1 during loading.

DETAILED DESCRIPTION OF THE INVENTION

A cold-gas pyrotechnic generator 1 according to the inventioncharacteristically comprises:

a) a casing 2 of revolution consisting of a base 10 and a cover 20fastened together transversely,

b) an ignition device 30,

c) elements 40 of a gas-generating pyrotechnic composition,

d) filtering means 50.

As shown in the figures, the base 10 substantially has the appearance ofa saucer and has an outer lateral wall 11 which may end in a flat rim12. This flat rim 12 will later serve as a fastening rim for thegenerator and will advantageously be provided with perforations 16intended to allow the passage of the members for fastening the generatorto the frame of the motor vehicle.

The base 10 also has a circular central opening 13 bordered by acylindrical, inner initial partition section 14. The base 10 moreoverhas a cylindrical, intermediate initial partition section 15 situatedbetween the inner initial partition section 14 and the outer lateralwall 11.

The cover 20 has an outer lateral wall 21 with the same thickness andthe same diameter as the outer lateral wall 11 carried by the base 10.This outer lateral wall 21 carried by the cover 20 comprises outerorifices 22 intended to allow the discharge of the gases generated bythe generator. The cover 20 moreover carries a cylindrical intermediatepartition 23 with the same thickness and the same diameter as theintermediate initial partition section 15 carried by the base 10.

The height of this intermediate partition 23 carried by the cover 20 issuch that it comes into contact with the initial partition section 15carried by the base 10 when the latter is assembled with the cover 20.The said intermediate partition 23 has intermediate orifices 24 intendedto allow the gases produced by the generator to pass through. As shownin the figures, the outer orifices 22 and the intermediate orifices 24are advantageously aligned so that they can be formed in a singleoperation.

According to another preferred embodiment of the invention, not shown inthe figures, the intermediate orifices 24 are closed by caps which areeasily broken under the influence of the rise in pressure of thegenerator. These caps may, for example, be made from aluminium foil.

The base 10 and the cover 20 preferably consist of a light metal such asaluminium or an aluminium alloy. These pieces may advantageously bemanufactured by cold forming technique. When they are fastened togethertransversely, the base 10 and the cover 20 together form a casing 2which characteristically establishes:

an inner central channel 80 communicating with the outside through thecircular central opening 13 carried by the base 10,

an annular peripheral chamber 60 between the lateral walls 11 and 21 onthe one hand and the intermediate partition 23 and the intermediateinitial partition section 15 on the other hand,

an annular intermediate chamber 70 between the peripheral chamber 60 andthe inner central channel 80.

Characteristically, and in a manner indispensable to the implementationof the invention, as will be explained below, the casing 2 of revolutiondefines, beween the inner central channel 80 and the annularintermediate chamber 70 a lateral opening 71 situated between the innerinitial partition section 14 carried by the base 10 and the cover 20,the height of this lateral opening 71 being greater than the largestdimension of the elements 40 described hereinafter of a pyrotechniccomposition.

It will, moreover, be observed that the intermediate annular chamber 70communicates with the peripheral chamber 60 through intermediateorifices 24 and that the peripheral chamber 60 communicates with theoutside through outer orifices 22.

According to the preferred embodiment shown in FIGS. 1 and 2, the cover20 comprises a circular central opening 26 with a diameter slightly lessthan the diameter of the central opening 13 carried by the base 10. Theopening 26 is bordered by an inner, cylindrical, initial partitionsection 25 defining a threaded central channel 80. In this case, thelateral opening 71 is between the inner initial partition section 14carried by the base 10 and the inner initial partition section 25carried by the cover 20. As shown in FIGS. 1 and 2, the said lateralopening 71 established between the inner channel 80 and the intermediateannular chamber 70 is preferably continuous.

The annular peripheral chamber 60 is employed as a cooling and filteringchamber and it contains filtering and cooling means 50 known to a personskilled in the art, such as grids and granulates of cooling agents suchas sodium bicarbonate. These filtering and cooling means 50 are held inplace by a flexible metal cover 61 which bears against the innersurfaces of the base 10 and the cover 20, as shown in the figures. Thismetal cover 61 has gas-admission orifices 62 which are preferablysituated in the bottom part of the said cover 61 bearing against thebase 10.

The annular intermediate chamber 70 is employed as a combustion chamberand contains elements 40 of a gas-generating pyrotechnic composition.These elements are preferably in the form of pellets, as shown in thefigures. Compositions based on sodium nitride will advantageously beemployed as the gas-generating pyrotechnic composition. The preferredcompositions within the scope of the present invention are compositionscontaining between 60% and 82% by weight of sodium nitride, between 15%and 35% by weight of molybdenum sulphide and between 2% and 9% by weightof sulphur. The compositions which are particularly preferred are thosewhich will have undergone a heat treatment at 120° C. intended toimprove their mechanical properties, as described in French PatentApplication 89.05032.

As shown in the figures, prefiltering grids 72 may be provided in thecombustion chamber 70, on the side of the intermediate orifices 24. Theelements 40 of a pyrotechnic composition are furthermore advantageouslyimmobilized, on the side of the lateral opening 71, by radiallyexpansible immobilization means 73. A metal wire mesh wound arounditself and placed resting against the inner initial partition sections14 and 25 so as to bear against the elements 40 of a pyrotechniccomposition will, for example, advantageously be employed as theimmobilization means.

An ignition device 30 is provided inside the inner central channel 80.The ignition device 30 shown in FIG. 1 comprises on the one hand a ring31 and on the other hand an igniter 32. The lateral wall of the ring 31comprises an upper part with an external screw thread and which may bescrewed onto the internal screw thread carried by the inner initialpartition section 25. The lateral wall of the ring 31 also comprises alower part which fits tightly, as a result of a sealing strip 35,against the inner surface of the inner initial partition section 14carried by the base 10. The lower part of the ring 31 comprises athreaded bore in which the igniter 32, which is an electricallytriggered igniter, may be screwed. In the generator shown in FIG. 1, theigniter 32 is a coaxially connected igniter, such as described in FrenchPatent 2,557,689. The ring 31 has a solid upper part 33 and is providedover its entire lateral wall with slots 34 situated facing the lateralopening 71 and with a height which is substantially equal to the heightof the latter.

The operation of the generator according to the invention is as follows.By an electric pulse, directed by the acceleration or decelerationdetector, the igniter 32 is started up and in turn causes the ignitionof the elements 40 of a gas-generating pyrotechnic composition. Thegases produced by the combustion of these elements enter the coolingchamber 60 through the intermediate orifices 24 after having optionallybroken the caps closing these orifices. The gases then enter thefiltering and cooling means 50 through orifices 62 carried by the metalcover 61 and, after filtering and cooling, leave the generator throughthe outer orifices 22 in order to inflate the pillow attached to thegenerator.

The invention thus enables a cold-gas pyrotechnic generator to beprovided with perfect symmetry of ignition and run-off of the gases.

Another essential advantage attained by the present invention lies inthe fact that the gas generator may only be loaded with pyrotechnicproducts after the base 10 and the cover 20 which constitute the casing2 have been assembled. The following procedure is used for thisoperation.

In a first stage, the casing 2 is formed by assembling the cover 20 withthe base 10 containing the filtering and cooling means 50. To do this,the filtering and cooling means 50, the cover 61, as well as, whereappropriate, the prefiltering grids 72 are arranged in the base 10 andassembly of the base 10 with the cover 20 is effected by any appropriatemeans. For the reasons indicated above, the preferred means of assemblywill, however, be welding of the base 10 with the cover 20 on the onehand along the contact surface between the outer lateral wall 11 and theouter lateral wall 21 and on the other hand along the contact surfacebetween the intermediate initial partition section 15 and theintermediate partition 23.

Since the base 10 and the cover 20 are bodies of revolution, the weldingis very advantageously carried out by friction or by inertia. Electronbeam welding is also well appropriate to this case. Whatever thetechnique chosen, the welding may, in any case, be carried out in theabsence of any pyrotechnic element, and therefore in absolutely safeconditions.

In fact the presence of the lateral opening 71, an essential element ofthe invention, situated between the combustion chamber 70 and the innercentral channel 80 and the height of which is greater than the largestdimension of the elements 40 of a pyrotechnic composition, enables thechamber 70 to be filled after the base 10 has been assembled with thecover 20.

In order to do this, in a second stage, the casing 2 is fastened onto arotating device such as a lathe and, after the casing has been set inrotation, the elements 40 of a pyrotechnic composition are poured intothe lateral opening 71 of the chamber 70 inside which they are held bythe centrifugal force communicated to the casing 2 by it being set inrotation.

It only remains for the ignition device 30 to be introduced in a thirdstage into the inner central channel 80, after having optionallyintroduced the immobilization means 73, in order to obtain the completegenerator according to the invention.

According to the preferred method of loading, shown in FIG. 2, thecasing 2 is placed on a lathe equipped with jaws, the axis A of thelathe being in the horizontal position. The casing is then rotated and afeed device 100, consisting of a cylindrical feed spout 101 with adiameter less than the diameter of the channel 80, is introduced intothe inner channel 80. This spout 101 is extended by a feed pipe 102which is inclined and connected to a feed hopper, not shown.

The spout 101 is closed with a sloping edge and is provided in its lowerpart with an opening 103 with a diameter greater than the diameter ofthe elements 40 of a pyrotechnic composition. This opening 103 issituated almost at the end of the spout 101 which is positioned suchthat the opening 103 faces the lateral opening 71 of the combustionchamber 70. The feed hopper then delivers a sufficient quantity ofelements 40 of a pyrotechnic composition in order to fill the combustionchamber 70. The elements 40 drop through the pipe 102 as far as thespout 101 from where they fall as a result of gravity into the chamber70 through openings 103 and 71. The elements 40 introduced into thechamber 70 are held pressed flat in the latter towards the intermediatepartition 23 under the influence of the centrifugal force communicatedto the casing 2.

The Applicant has noted that the speed of rotation of the casing 2 mustbe equal to at least 100 revolutions per minute and that a speed of 300revolutions per minute is generally sufficient for elements of apyrotechnic composition in pellet form and for the generators which areusually employed in the automobile industry. Moreover, the Applicant hasalso observed that a better filling coefficient of the combustionchamber 70 with elements 40 is obtained if, during loading, the casing 2is not only set in rotation but that it is also subjected to vibrations.Such vibrations, preferably longitudinal, may, for example, be obtainedby placing an electromagnetic vibrating plate between the jaws of thelathe and the casing 2.

When the loading with elements 40 is completed, the feed device 100 iswithdrawn, the casing 2 being maintained in rotation. The metal wiremesh 73, wound around itself, is then introduced into the inner channel80 from where it is inserted into the lateral opening 71, while allowingit to expand radially.

The lathe is then stopped and it then only remains, in a last stage, toscrew in the ring 31, and then the igniter 32, in order to obtain thecomplete generator.

We claim:
 1. A method of manufacturing a cold-gas pyrotechnic generatorof the type having a casing of revolution having a central axis, saidcasing being provided with outer orifices and including a base and acover fastened together transversely, said base and said cover togetherforming:an inner ignition channel for receiving an ignition device; anannular intermediate chamber employed as a combustion chamber orcontaining elements of a gas-generating pyrotechnic composition; anannular peripheral chamber employed as a cooling and filtering chamber,said peripheral chamber containing filtering means;wherein saidintermediate chamber and said peripheral chamber are in communicationthrough intermediate orifices therebetween, and said peripheral chamberis in communication with the outside through said outer orifices, andwherein said casing defines, between said inner central ignition channeland said intermediate chamber, a lateral opening whose height is greaterthan the largest dimension of said elements of gas-generatingpyrotechnic composition, wherein said lateral opening is continuousabout said central axis, the method comprising the steps of a) placingthe filtering means in the chamber and assembling the cover with thebase, b) rotating the casing about the central axis thereof, c) pouringthe pyrotechnic composition into the lateral opening of the chamberwhile rotating the casing, and d) introducing an ignition device intothe inner central channel.
 2. The method according to claim 1, wherein,in step a), assembling of the cover with the base is carried out bywelding.
 3. Method according to claim 2, characterized in that thewelding is obtained by friction.
 4. The method as claimed in claim 1,wherein step b) is carried out by fastening the casing on ahorizontal-spindle lathe.
 5. Method according to claim 4, characterizedin that the speed of rotation of the casing (2) during loading isgreater than 100 revolutions per minute.
 6. Method according to claim 5,characterized in that, during loading, the casing (2) is subjected tovibrations.